The present invention relates to a magnetic head in which a step-up transformer is integrally assembled and, more particularly, to a magnetic head having a video head shape.
Hitherto, a head arrangement in which a step-up transformer is coupled to an inductive head of one turn (or a few turns) has been known. This kind of magnetic head is shown in FIG. 1. According to such an arrangement, by extremely reducing the number of signal windings and miniaturizing the magnetic circuit interlinked therewith, the output per turn is increased. The low absolute output due to a small number of windings is compensated by the step-up transformer. FIG. 2 shows a conventional example which is suitable for use as a video head. In FIG. 2, reference numerals 1 and 2 denote magnetic head cores which confront one another to form a head gap 3; and 4 indicates a half body of a transformer core. This half body is disposed to face the rear portion of the head core 1, thereby forming a transformer core of a substantially ring-shaped magnetic path. A metal layer 5 is embedded into a groove formed to divide the head core 2 into a front gap portion and a back gap portion. A coil 6 of one turn passes through a central groove 7 of the transformer core 4. The edge portion of the coil 6 is connected to the edge portion of the metal layer 5, thereby forming a closed loop coil of one turn which interlinks the head magnetic circuit and transformer magnetic circuit. A secondary coil 8 of the transformer is provided. In this manner, a head arrangement which is equal to FIG. 1 is obtained.
According to such a head structure, the head core portion and the one-turn winding are provided in a so-called bulk constitution. Consequently, such a head arrangement has the following problems. Namely, the one-turn inductance and DC resistance of the head cannot be sufficiently reduced by sufficiently decreasing the size of the head magnetic circuit and one-turn winding. Further, since the arrangement of the primary and secondary coils of the transformer section is not proper, a sufficiently high coupling coefficient of the transformer cannot be obtained. The following explains why the above problems obstruct realization of a highly efficient head.
A circuit shown in FIG. 3 will now be considered as a simple equivalent circuit of the head of FIG. 1. The DC resistance and stray capacitance of the secondary coil are omitted in this circuit. L.sub.1 and L.sub.2 denote self-inductances of the primary and secondary coils of the transformer, respectively. M denotes a mutual inductance between the primary and secondary coils; L.sub.H is an inductance of the one-turn head; r a DC resistance of one turn including both the winding of the head and the winding of the primary coil of the transformer; E.sub.i a reproduced voltage which is induced in the head winding; and E.sub.0 a secondary output of the transformer.
The significant factors in the actual use of such a head are the inductance L, real part Re(Z) of the impedance and transfer efficiency G=E.sub.0 /E.sub.i of the whole structure including the head and the transformer.
The high frequency operational range of the head is determined by L. When the frequency is sufficiently high, the value of L can be calculated by the following expression from FIG. 3. EQU L=L.sub.H .multidot.N.sup.2 +L.sub.2 (1-K.sup.2) (1)
where N is a number of secondary coil windings (i.e., step-up ratio) of the transformer and K is a coupling coefficient of the transformer. Although the first term (L.sub.H .multidot.N.sup.2) of expression (1) is the inductance which is inevitably generated irrespective of the performance of the transformer, the second term is the amount which is added when the transformer is not ideal. Since it is necesary to set N to a value as large as possible for the value of L which can be allowed in use, L.sub.H needs to be set to a sufficiently small value. In addition, it is necessary to minimize the second term of expression (1) by setting K to a value sufficiently close to "1" by increasing the coupling degree of the transformer.
Next, the impedance noise is determined by Re(Z) and a lower value of Re(Z) is better. At a frequency higher than the low-band cut-off frequency, Re(Z) can be calculated by the following expression. ##EQU1## Assuming that L.sub.H is smaller than L.sub.1, expression (2) can be approximated by the following expression. ##EQU2## The one-turn resistance value r on the primary side is nearly transformed into the value multiplied by the square of the step-up ratio, i.e., rN.sup.2 on the secondary side. Therefore, r needs to be set to a sufficiently low value.
Further, the transfer efficiency G becomes ##EQU3## at a frequency above the low-band cut-off frequency. Assuming that L.sub.H is smaller than L.sub.1 similarly, EQU G.perspectiveto.K.multidot.N (5)
The transfer efficiency is reduced by the value of K from the step-up ratio N. Therefore, K needs to be set to a value sufficiently close to "1".
To obtain the high performance characteristics of the head including the step-up transformer as mentioned above, it is required that the inductance and DC resistance of the one-turn portion of the head be sufficiently small and the coupling coefficient of the transformer be sufficiently close to "1". These conditions are not fully satisfied in the conventional example.